Warpage

When plastic material is injected into a mold cavity, it is cooled and solidifies to from a part that is made to final part dimensions.However, if the cooling of this part in the mold is not uniform, differences in part shrinkage can occur, causing the part to distort.This condition is referred to in the injection molding industry as warpage. Warpage is also the result of excessive stresses that are built into a part during the molding process.

Causes:

1. An injection speed that is too fast will increase the melt temperature, forcing the melt to overpack the cavity. This overpacking creates molded-in stresses in the part, which warp the part

2. The injection speed is too slow, the viscosity of the material increases the amount of packing pressure needed to fill the cavity. This higher packing pressure adds molded-in stresses in the part, causing warpage

3. Long injection forward times contribute to part warpage by allowing more time for more material to overpack the cavity

4. Check ring wear can cause shot-to-shot consistency, creating overpacked parts

5. If the gate has not sealed off before the cavity is full, overpacking occurs, causing warpage

6. If not enough gates are used to fill a part, higher packing pressures occur to fill out the part, producing molded-in stresses

7. Smaller gate sizes cause a high degree of molecular orientation in the polymer that causes different shrinkage in different directions of the part (machine vs. transverse)

8. Insufficient cooling times do not provide enough time for the part to cool Increasing shrinkage

9. Running the mold halves at different temperatures can cause shrinkage in one direction to help overall warpage. Parts have a tendency to warp toward hotter mold halves since this causes higher shrinkage

10. If ejection forces are uneven due to ejector pin placement or length, stresses can be induced in the part causing warpage

11. Differences in wall thickness promote warpage since thin sections cool slower

12. Sharp comers can promote molded-in stresses that can cause warpage

Solutions:

1. Decreasing the melt temperature if the injection speed is too fast

2. Increasing the injection speed to decrease the holding pressure needed to fill the cavity

3. Limiting the injection forward time

4. Replacing check ring

5. The gate seal needs to be measured to assure that the cavity is not full before gate seal takes place

6. Increasing the number of gates reduces the packing pressure by distributing the pressure over a

number of gates

7. Larger gate sizes reduce orientation

8. Increasing cooling time or fixturing the part are options to prevent the part from distorting

9. Location and placement of cooling lines should be reviewed to ensure that even efficient cooling is

taking place to the part

10. Improving or modifying pin design can reduce

11. Uniform wall thickness helps avoid warpage part warpage

12. Removing sharp corners can reduce molded-in stresses and reduce stresses that can be caused

during part ejection